The basic features of a typical and well known fluorescent lamp circuit, shown in FIG. 1, are important background information with respect to the present invention. A fluorescent lamp 10 is connected in series with a current limiting inductor known as a ballast 12. Conventional alternating current (AC) power from a source 14 is applied to the series connected lamp 10 and ballast 12. The fluorescent lamp 10 is formed generally of an evacuated translucent housing 16 which has two electrodes known as cathodes 18 placed at opposite ends of the housing 16. A small amount of mercury is contained within the evacuated housing 16. When the lamp 10 is lighted, an ionized plasma of vaporized mercury conducts power between the cathodes 18. Because of the high conductivity or low resistance characteristics of the mercury plasma, the ballast 12 is necessary to limit the current flow from the source 14 through the plasma, to prevent the cathodes 18 from burning out.
A starter 20 is connected between the cathodes 18. The function of the starter 20 is to light the lamp 10, which may prove difficult or impossible in certain circumstances. For example, the mercury inside the housing 16 may be condensed in a liquid state. Before the mercury can be ionized as the plasma, it must first be vaporized. Low temperature ambient conditions may make it difficult to vaporize the liquid mercury.
To initiate lighting of the lamp 10, the starter 20 first heats the cathodes 18. The starter 20 establishes a closed circuit between the cathodes 18 for a period of time during which the current flows through both cathodes and the starter and heats the cathodes. The heat from the cathodes helps vaporize the mercury within the housing 16. The heated cathodes 18 also emit low work energy ions from material coated on the surface of the cathodes. The emitted ions create an ionized cloud surrounding each cathode 18. This ionized cloud assists in establishing a break-over arc between the cathodes 18 to start the lamp 10 and to maintain it lighted.
After heating the cathodes 18, the starter 20 opens the circuit conducting current through the cathodes 18. The current flow terminates almost instantaneously, causing a relatively high change in current in a relatively short amount of time (di/dt). The ballast 12 responds to the relatively high di/dt by producing a very high voltage pulse 22 as shown in FIG. 2. In a typical fluorescent lamp circuit powered by a conventional 120 volt RMS source 14, the voltage pulse will typically be in the range of 400 to 700 volts.
The pulse 22 is of sufficiently high voltage to break down the ionized electron cloud and the mercury vapor within the housing 16, thereby conducting an arc between the cathodes 18. The arc jumps directly between the cathodes 18 because the starter 20 has opened and no longer presents a current path between the cathodes. The current of the arc creates a plasma to light the lamp 10. The current flow through the plasma between the cathodes 18 thereafter continues to heat the cathodes 18. The heated cathodes are sufficient to maintain enough ionization to allow the normal AC voltage from the source 14 to ignite the plasma during the subsequent half cycles of applied AC voltage 24, shown in FIG. 2, without the need for further high voltage starting pulses 22. The plasma emits ultraviolet light which interacts with phosphorus placed on the interior of the housing 16, and the phosphorus emits visible light.
The typical voltage characteristics applicable to the fluorescent lamp 10 are shown in FIG. 2. The applied voltage from the conventional AC power source 14, such as a 60 hertz 120 volt RMS signal, is shown at 24. Under operating conditions, the voltage across the cathodes 18 builds up until an ignition or break-over voltage 26 is reached, at approximately 125 volts. The ignition voltage may vary somewhat depending on the heat of the cathodes and the extend of vaporization, but the voltage 26 necessary to sustain the plasma state remains approximately constant after steady state conditions are attained. Because the 177 volt peak voltage of the 120 volt RMS signal is considerably greater than the ignition voltage 26, the current between the cathodes 18 through the plasma will increase to an unacceptable level unless the ballast 12 is employed. The ballast 12 limits the current under plasma ignition conditions.
One well known type of starter 20 is a simple push button mechanical starter switch. The user holds the switch closed for a short time period to allow the cathodes 18 to heat and then at some random time releases the starter switch. If the cathodes 18 are sufficiently heated and if the starter button is released when the applied AC voltage 24 across the cathodes 18 is at or above the ignition voltage 26, the lamp 10 will light. If the right combination of cathode heat and the starter switch release point does not occur, an additional attempt to light the lamp 10 is required. The disadvantage of the mechanical switch starter is that it requires manual intervention, at least once and maybe many times, to light the lamp 10.
Another well known type of starter 20 is known as a "glow bottle". A glow bottle is an evacuated housing within which there are positioned a radioisotopic ionizable gas and a bimetal switch. The glow voltage of the radioisotopic gas is above the level of the lamp ignition voltage 26 shown in FIG. 2. When the fluorescent lamp 10 is not lighted the full voltage of the source 14 is impressed across the glow bottle. The radioisotopic gas breaks down, begins to glow and heats the bimetal switch. When the bimetal switch becomes hot enough, it closes and shunts the voltage away from the radioisotopic gas in the glow bottle and conducts current through the cathodes 18 to heat them. The radioisotopic gas starts cooling when the bimetal switch closes, causing the bimetal switch itself to begin to cool.
When the bimetal switch has cooled sufficiently, it opens and causes a high di/dt. The ballast 12 responds to the di/dt by applying the high voltage pulse 22 to the warmed cathodes 18. The lamp 10 will only be lighted if the bimetal switch opens at a time when the AC voltage 24 across the cathodes 18 is above the ignition voltage 26. Once the fluorescent lamp is lighted, the voltage across the gas in the glow bottle never reaches a high enough value to cause the radioisotopic gas to glow, because the ignition voltage 26 is lower than the ionization voltage of the radioisotopic gas. Once the lamp is extinguished, the glow bottle will again become operative.
One of the advantages of the glow bottle is that it is self-starting. Any time that the lamp 10 extinguishes, the applied voltage from the AC source is applied to the radioisotopic gas to make it glow, and the operation described above occurs. One of the primary disadvantages of the glow bottle is the random and long time delay in igniting the fluorescent lamp when the power is first applied to it. The delay while the glow bottle functions may result in frustration to the user who expects immediate light when the light switch is closed. Another disadvantage to the glow bottle is that good regulation of the applied voltage from the source 14 is required to break down the radioisotopic gas under the proper conditions and to prevent it from breaking down during times when the lamp is lighted.
The voltage regulation of power delivery in some parts of the world makes it difficult or impossible to use glow bottle starters or indeed even fluorescent lamps. It is also difficult to use fluorescent lamps with manual starters in circumstances of frequent momentary or longer power interruptions because the lamp must be manually restarted after each interruption. Unfortunately, the economic circumstances which give rise to the power delivery difficulties are usually the same economic circumstances where more lighting which consumes less electrical energy would be of great benefit. Combined with the difficulties that low ambient temperatures pose for starting or igniting fluorescent lamps, the convenient and successful applications of fluorescent lamps may be limited. Many of these difficulties are directly attributable to the shortcomings of the typical fluorescent lamp starter.
Attempts to improve the functionality and reliability of starters have included the use of semiconductor electronic circuits. One of the significant difficulties with a semiconductor starter circuit has resulted because of the relatively high voltage pulses 22 will destroy most common semiconductors such as bipolar junction transistors, FETs and the like. Some semiconductor devices such as MOSFETs and triacs have deeply diffused junction profiles and therefore capable of withstanding very high voltages, but may be expensive and difficult to employ in numbers which are economical or difficult to incorporate in an integrated circuit.
It is with respect to this and other background information that the present invention has evolved.